Method for detecting the magnetic flux the rotor position and/or the rotational speed

ABSTRACT

The method serves for detecting the magnetic flux, the rotor position and/or the rotational speed of the rotor in a single- or multiphase permanent magnet- or -synchronous motor or -generator using the stator voltage equations  
       L·{dot over (i)}   α   =−R·i   α   +p·ω·ψ   mβ   +u   α   Equation (1)  
       L·{dot over (i)}   β   =−R·i   β   −p·ω·ψ   mα   +u   β   Equation (2)  
     It is characterized in that with the evaluations one takes into account the energy conditions of the rotor, by which means one may achieve an accuracy which is considerably greater than known methods.

BRIEF DESCRIPTION OF THE INVENTION

[0001] The invention relates to a method for detecting the magnetic flux, the rotor position and/or the rotational speed of the rotor in a single-phase- or multi-phase-permanent magnet or -synchronous motor or -generator according to the features specified in claim 1.

[0002] Magnetic flux, rotor position and rotational speed are defined by the stator voltage equations known per se:

L·{dot over (i)} _(α) =−R·i _(α) +p·ω·ψ _(mβ) +u _(α)  Equation (1)

L·{dot over (i)} _(β) =−R·i _(β) −p·ω·ψ _(mα) +u _(β)  Equation (2)

[0003] in which

[0004] L is the inductance

[0005] i_(α) the current in direction α

[0006] i_(β) the current in direction β

[0007] {dot over (i)}_(α) the derivative with respect to time of the current in direction α

[0008] {dot over (i)}_(β) the derivative with respect to time of the current in direction β

[0009] R the ohmic resistance

[0010] p the pole pair number

[0011] ω the rotational speed of the rotor

[0012] ψ_(mα) the magnetic flux in the direction α

[0013] ω_(mβ) the magnetic flux in the direction β

[0014] u_(α) the voltage in direction α

[0015] u_(β) the voltage in direction β,

[0016] As is deduced from these equations, the previously mentioned variables may be evaluated if the voltage and current in the directions α and β are known. The latter may also be evaluated as electrical data in a simple manner. According to the state of the art, this is only possible if the magnitude of the magnetic flux is assumed to be constant since otherwise the equation system may not be unambiguously solved due to too many unknown variables. Since the magnetic flux is indeed not constant, but the magnitude varies with time and rotor position, this known method is erroneous, which leads to the fact that it is only suitable in a limited manner for control and regulation processes of the motor.

[0017] Today modern multi-phase permanent magnet motors are often provided with power electronics, i.e. the commutation is effected electronically. For the control however the knowledge of the current rotor position is very significant, not only to be able to operate the motor with a high efficiency, but also in order to protect the sensitive components of the power electronics and to achieve an improved dynamic behavior of the drive.

[0018] The measurement of the rotational speed may however be effected via an external measurement arrangement in a comparatively simple manner. The exact evaluation of the rotor position however is complicated.

[0019] On the other hand where possible one tries to evaluate these values by calculation since on account of the digital electronics which are regularly present in the control and regulation part of the motor electronics, suitable computing power is available or may be made available with little effort. Suitable programs for calcuated evaluation could also be integrated by software implementation without much effort.

[0020] Against this background it is the object of the invention to provide a method of the known type for detecting the magnetic flux, the rotor position, and/or the rotational speed of the rotor in a single-phase or multi-phase-permanent magnet motor or -synchronous motor or -generator.

SUMMARY OF THE INVENTION

[0021] This object is achieved by the features specified in claim 1. Advantageous embodiments of the invention are specified in the dependent claims as well as the subsequent description.

[0022] The basic concept of the present invention is to apply the stator voltage equations known per se with a method for detecting the previously mentioned variables, but however in contrast to the state of the art not to set the magnetic flux constant, but to also include the energy relations in the magnet of the rotor in order thus to be able to determine the previously mentioned variables, in particular the rotor position or its derivative with respect to time and rotational speed in a more exact manner.

[0023] The present invention may be applied to single-phase- as well as multi-phase permanent magnet or -synchronous motors as well as corresponding -generators. Inasmuch as it concerns single-phase motors and generators, one of the two stator voltage equations drops away. Otherwise with two-phase or multi-phase motors or generators one computes with the stator voltage equation for two-phase motors and generators, wherein with three-phase and multi-phase motors one reduces or transforms by calculation to a two-phase model in a manner known per se. Inasmuch as this is concerned then values detected with measurement technology must be accordingly converted to a two-phase model.

[0024] The present invention in particular is envisaged for permanent magnet motors, but in the same manner may also be applied to synchronous motors or generators, wherein with synchronous motors or generators the magnet formed by the rotor coil takes the place of the permanent magnet. In this context the generator application may also be the case in combination with the control of motors operated with power electronics if with generator operation these supply to the mains in order to determine the rotor position of the mains generator.

[0025] The method according to the invention may also be used for generators, for example with the control.

[0026] In FIG. 1 there is shown such a two-phase permanent magnet motor, there are provided two phases α and β in a motor which are shifted by 90° to one another which are symbolized by two coils 3 and 4. Within this stator 1 there is arranged a rotor which comprises a permanent magnet 5 with a diametrical polarity distribution N and S which is rotatingly mounted within the stator 1.

[0027] In order to take into account the energy conditions in the magnet 5 of the rotor 2 the following equations (3) and (4) are applied.

{dot over (ψ)}_(mα) =−p·ω·ψ _(mβ)  Equation (3)

{dot over (ψ)}_(mβ) =p·ω·ψ _(mα)  Equation (4)

[0028] wherein

[0029] {dot over (ψ)}_(mα) is the derivative with respect to time of ψ_(mα) and

[0030] {dot over (ψ)}_(mβ) the derivative with respect to time of ψ_(mβ.)

[0031] The particularity of these rotor energy equations lies in the fact that the magnetic flux in the β-direction enters the derivative with respect to time of the magnetic flux in the α-direction and vice versa.

[0032] By way of this there results a computational motor model with which for example as illustrated by way of FIG. 2 one may determine electrical, magnetic and/or mechanical values of the motor.

[0033] In the following, in the motor models represented as block diagrams a computed value is in each case represented by {circumflex over ( )} whereas with the values which are not characterized by {circumflex over ( )} it is the case of measured values.

[0034] It is to be understood that from the initially mentioned variables (magnetic flux, rotor position, rotational speed) one may in each case evaluate one if one uses the motor model symbolized by the block 6 in FIG. 2. This motor model symbolized by the block 6 consists of the equations (1) to (4) with which one of the previously mentioned values may be evaluated by calculation in a relatively exact manner.

[0035] With the method according to FIG. 2 the voltages u_(α) und u_(β), i.e. the stator voltages in the α and β direction are measured or computed in another manner, just as ω the rotational speed. These variables are substituted into the equations (1) and (4), so that one may evaluate by calculation the speed of the magnetic flux ω_(flux), the motor currents i_(α) in the direction α and i_(β) in the direction β as well as the magnetic flux ψ_(α) in direction α und ψ_(β) in the direction β. The corresponding values evaluated by calculation are characterized by {circumflex over ( )}:

L·{circumflex over ({dot over (i)})} _(α) =−R·î _(α) +p·{circumflex over (ω)}·{circumflex over (ψ)} _(mβ) +u _(α)  Equation (1)

L·{circumflex over ({dot over (i)})} _(β) =−R·î _(β) −p·{circumflex over (ω)}·{circumflex over (ψ)} _(mα) +u _(β)  Equation (2)

{circumflex over ({dot over (ψ)})}_(mα) =−p·{circumflex over (ω)}·{circumflex over (ψ)} _(mβ)  Equation (3)

{circumflex over ({dot over (ψ)})}_(mβ) =p·{circumflex over (ω)}·{circumflex over (ψ)} _(mα)  Equation (4)

[0036] From the magnetic flux ω_(α) in the direction α and ω_(β) in the direction β then by way of an angle calculator 7 which applies the geometric designation according to $\begin{matrix} {\rho = {\frac{1}{p} \cdot {{Arctg}\left( \frac{\psi_{m\quad \beta}}{\psi_{m\quad \alpha}} \right)}}} & {{Equation}\quad (5)} \end{matrix}$

[0037] one may evaluate the position ρ of the magnetic flux. In this basic motor model 6 the rotor position is determined by equating with the position of the magnetic flux assuming that these always agree in a real manner.

[0038] Since this motor model 6 in its simplest form only represents an approximation by calculation of the actual values, it may be improved by further measures. Such an improvement for example is represented by way of the method shown in FIG. 3. As FIG. 3 shows here the basic motor model 6 consisting of the equations (1) and (4) is taken as a basis, wherein the stator voltages u in directions α und β, u_(α) und u_(β) as well as the rotor speed ω flow into the model for example as measured variables. In contrast to the method according to FIG. 2 in the model 6 a according to FIG. 3 however the stator current in the α- und β-direction, thus i_(α) and i_(β) are additionally determined, joined by subtraction with the calculated current values î_(α) and î_(β) determined by the motor model 6 a (this is represented in FIG. 3 the subtraction junction 8) and the value resulting therefore is led to a correction term 9 which also flows into the motor model 6 a in a correcting manner. In this manner one creates a refined motor model 6 a and thus an improved method for evaluating the previously mentioned values which consists of the equations (1a), (2a), (3a) und (4a):

L·{dot over (i)} _(α) =−R·i _(α) +p·ωψ _(mβ) +u _(a)+υ_(1α)  Equation (1a)

L·{dot over (i)} _(β) =−R·i _(β) −p·ω·ψ _(mα) +u _(β)+υ_(1β)  Equation (2a)

{dot over (ψ)}_(mα) =−p·ω·ψ _(mβ)+υ_(2α)  Equation (3a)

{dot over (ψ)}_(mβ) =p·ω·ψ _(mα)+υ_(2β)  Equation (4a)

[0039] in which υ_(1α), υ_(1β), υ_(2α), υ_(2β) are correction terms.

[0040] In the method according to FIG. 3 the measured stator currents in the α and β-direction in comparison to the computed currents in the α and β direction are provided as a correction term. It is to be understood that this is only to be understood as an example, in the same manner the motor currents may flow into the motor model 6 or 6 a and the motor voltages evaluated by calculation and as the case may be inputted as a correction term by comparison to the actual voltages. One may also provide several correction terms which are constructed based on several electrical variables.

[0041] For the previously described method represented as an example by way of FIG. 3 there thus for example results the following equations

L·{circumflex over ({dot over (i)})} _(α) =−R·i _(α) +p·{circumflex over (ω)}·{circumflex over (ψ)} _(mβ) +u _(α)+υ_(1α)  Equation (1a)

L·{circumflex over ({dot over (i)})} _(β) =−R·i _(β) −p·{circumflex over (ω)}·{circumflex over (ψ)} _(mα) +u _(β)+υ_(1β)  Equation (2a)

{circumflex over ({dot over (ψ)})}_(mα) =−p·{circumflex over (ω)}·{circumflex over (ψ)} _(mβ)υ_(2α)  Equation (3a)

{circumflex over ({dot over (ψ)})}_(mβ) =p·{circumflex over (ω)}·{circumflex over (ψ)} _(mα)+υ_(2β)  Equation (4a)

[0042] in which υ_(1α), υ_(1β), υ_(2α), υ_(2β)0 are correction terms

[0043] wherein the correction terms are formed by a correction factor and the difference of the computed electrical values and the measured electrical values as follows:

υ_(1α) =K _(i)·(î _(α) −i _(α))

υ_(2α) =−K _(ψ)·(î _(β) −i ₆₂ )

υ_(1β) =K _(i)·(î _(β) −i _(β))

υ_(2β) =K _(ψ)·(î _(a) −i _(a))

[0044] As is evident from the above equations the correction terms υ₂ are formed such that in the equations (3a) and (4a) in the one phase they are formed by way of the difference between computed and measured currents of the other phase. The variables K_(i) und K_(ψ) at the same time in each case form a constant factor.

[0045] By way of example in FIG. 4 there is shown a further embodiment of the method according to the invention with which apart from the corrected motor model 6 a according to FIG. 3 there is shown a further development in which the rotational speed of the rotor ω is evaluated by calculation. With the motor models according to the FIG. 2 and 3 the rotor rotational speed ω is entered as an input variable. The rotational speed is then usually detected sensorically, and specifically preferably with the help of a Hall sensor, as this is also known per se.

[0046] There are however constellations with which the rotor rotational speed may also be determined by calculation or with which the sensorically determined readings are not sufficiently accurate or are temporally available only in comparatively large intervals. For these cases in a further development of the invention there is provided an adaptation block 10 which by way of a rotational speed correction term 11 in which the difference between an assumed or computed rotational speed and the flux speed ω_(flux) computed from the motor model 6 a is formed, the evaluated rotational speed approximates the actual rotor speed until the rotational speed correction term 11 assumes the value zero. This correction term 11 is shown in FIG. 4 as a result of the subtractory junction effected in the node point 14 and proceeds from the assumption that the speed of the magnetic flux and the rotor speed must always agree. In the adaptation block 10 then the difference evaluated by way of the rotational speed correction term 11 where appropriate taking account of a correction factor is added to the previously evaluated rotational speed and is outputted as a new computed rotational speed. This new computed rotational speed then on the one hand is inputted into the motor model 6 a and on the other hand appears at the node point 14 which on account of the new rotational speed inputted into the motor model 6 a also obtains a new speed of the magnetic flux and by way of this emits a new rotational speed correction term 11 which again initiates the previously described approximation process by the adaptation block 10 until finally the correction term 11 assumes the value zero, thus the speed of the magnetic flux as is evaluated from the motor model 6 a, and the rotor speed, thus the computed rotational speed of the rotor agree.

[0047] Within the motor model 6 a the speed of the magnetic flux is formed by the derivative with respect to time of the evaluated position of the magnetic flux. If one thus differentiates the equation (5) with respect to time in or to obtain the speed of the magnetic flux and if one substitutes the equations (3a) and (4a) in this differentiated equation (5) then the speed of the magnetic flux results as follows: $\begin{matrix} {{\overset{\overset{.}{\hat{}}}{\rho} = {\omega_{Flux} = {\hat{\omega} + {\frac{1}{p} \cdot \frac{{\upsilon_{2\quad \beta} \cdot {\hat{\psi}}_{m\quad \alpha}} - {\upsilon_{2\quad \alpha} \cdot {\hat{\psi}}_{m\quad \beta}}}{{\hat{\psi}}_{m\quad \alpha}^{2} + {\hat{\psi}}_{m\quad \beta}^{2}}}}}}\begin{matrix} {{wherein}\quad {\frac{1}{p} \cdot \frac{{\upsilon_{2\quad \beta} \cdot {\hat{\psi}}_{m\quad \alpha}} - {\upsilon_{2\quad \alpha} \cdot {\hat{\psi}}_{m\quad \beta}}}{{\hat{\psi}}_{m\quad \alpha}^{2} + {\hat{\psi}}_{m\quad \beta}^{2}}}\quad {represents}\quad {the}\quad {rotational}\quad {speed}} \\ {\quad {{correction}\quad {term}\quad 11}} \end{matrix}} & {{Equation}\quad (6)} \end{matrix}$

[0048] The adaptation block 10 forms part of an approximation process with which the assumed and computed rotational speed is brought into agreement with the actual rotor rotational speed with the help of the motor model 6 a and the rotational speed correction term 11, until the rotational speed correction term becomes zero.

[0049] Additionally one may take into account the difference between the rotor rotational speed computed in the adaptation block 10 and a measured rotor rotational speed. Such an additional rotational speed correction term 15 is additively combined at the node point 12 with the rotational speed correction term 11, which as a formula is represented as follows:

Δω_(mess) =K _(v)·({circumflex over (ω)}−ω_(Rotor))   Equation (7)

[0050] wherein

[0051] Δω_(mess) forms the additional rotational speed correction term and K_(v) a constant.

[0052] If no measured rotor rotational speed is available this additional rotational speed correction term 15 is equal to zero. K_(v) represents an amplification factor with which this additional rotational speed 15 is inputted.

[0053] According to FIG. 5 one may also determine the rotational speed by way of a system rotational speed change correction term 13 which may be derived from the rotational speed model. The method is different from that described previously by way of FIG. 4 in that apart from the adaptation block 10 there occurs a system rotational speed change correction term 13 derived from the rotational speed model.

[0054] The rotational speed model contains further information on mechanical relationships of the drive system. Usefully the change of the rotational speed, thus the temporal change in the rotor speed is expressed by a mechanical condition equation which takes into account the previously mentioned relationships. The change of the rotational speed may at the same time be taken into account by the following equation in the rotational speed model 13: $\begin{matrix} {\overset{.}{\omega} = {\frac{1}{J} \cdot \left( {M - M_{L}} \right)}} & {{Equation}\quad (8)} \end{matrix}$

[0055] in which

[0056] M is the driving moment

[0057] M_(L) the load moment

[0058] J the moment of inertia of the rotating load

[0059] This condition equation which is known per se indicates that the change in rotational speed is only effected if the drive moment is larger than the load moment or vice versa, and that this change is then dependent on the difference moment as well as the moment of inertia of the rotating load.

[0060] This additional information in combination with the adaptation block 10 with a changing rotational speed leads to a quicker result with which the computed rotational speed of the rotor corresponds to the actual rotational speed of the rotor and is thus suitable in particular for tasks with a highly dynamic drive. The rotational speed model however assumes that corresponding mechanical or electrical variables are for example available by way of measurement or in another manner. At the same time, as the case may be, the rotational speed model may also simplified by skillfully met assumptions. If the motor for example runs at constant speed and the rotational speed model is used for determining the rotational speed the equation (8) results in zero so that then the rotational speed model is not used in its true sense but instead of this the rotational speed is evaluated as described by way of FIG. 4. The assumption that the motor runs at a constant rotational speed therefore does not go further than that described by way of FIG. 4.

[0061] The equation (8) may on the other hand be simplified by certain load assumptions, for example by the load condition M_(L)=0 or constant. The load moment is often not known or may only be determined with great difficultly. In many cases however one may assume a constant load moment. With this assumption the system rotational speed change correction term 13 then has the following form: $\begin{matrix} {{\Delta \quad \omega_{System}} = {{\frac{1}{J} \cdot \left( {M - K_{4}} \right)}\quad {wherein}}} & {{Equation}\quad (9)} \end{matrix}$

[0062] Δω_(System)—is the system change correction term and

[0063] K₄—the constant.

[0064] The constant K₄ is zero if the load moment is assumed to be zero. Otherwise the constant K₄ is to be previously determined for the respective unit type or application.

[0065] The drive moment is determined by the equation (10):

M=K ₂·(ψ_(mα) ·i _(β)ψ_(mβ) ·i _(α)),   Equation (10)

[0066] in which K₂ is a constant.

[0067] The term in brackets in equation (10) is already known from the motor model 6 a. If one substitutes equation (10) into equation (9) then it becomes evident that for this case (assumption that the load moment is zero or constant) the system change correction term 13 may be computed from the motor model 6 a. Thus without further measurement one may determine this correction term 13 and compute the rotational speed of the rotor more quickly and accurately. It is thus particularly favorable if the drive moment may be determined from the variables deduced from the motor model 6 a.

[0068] If the motor for example is applied in a centrifugal pump unit then the load moment may be determined by calculation in a simple manner since it is evaluated by the equation (11):

M _(L) =K ₁·ω²,   Equation (11)

[0069] in which K₁ is a constant,

[0070] which provides a relationship for the rotor rotational speed. Here too the variables derived from the motor model may be entered into the rotational speed model without further mechanical or electrical measurements being required.

[0071] Irrespective of whether the rotational speed is only determined by way of the adaptation model 10 or supplementary to this also by taking into account the rotational speed model, a measured rotational speed may also be inputted in order to obtain the result more quickly or to increase the accuracy of the computed values. Such a quick and accurate acquisition of the motor operation variables as may be effected by way of the previously mentioned inventive method is a precondition for a dynamic and stable activation of the motor.

[0072] The previously described methods may be implemented into digital motor electronics without further ado. The continuous detection and storage of the corresponding electrical values of the motor, thus of the motor currents and voltages today belongs to the state of art applied today. This data thus on the control side is available anyway so that the present invention, as the case may be, may be used within the digital motor control in order to improve this. 

What is claimed is:
 1. A method for detecting the magnetic flux, the rotor position and/or the rotational speed of the rotor in a single- or multi-phase permanent magnet motor or -synchronous motor or -generator using the stator voltage equations: L·{dot over (i)} _(α) =−R·i _(α) +p·ω·ψ _(mβ) +u _(α)  Equation (1) L·{dot over (i)} _(β) =−R·i _(β) −p·ω·ψ _(mα) +u _(β)  Equation (2) in which L is the inductance i_(α) the current in the direction α i_(β) the currect in the direction β {dot over (i)}_(α) the derivative with respect to time of the current in the direction α {dot over (i)}_(β) the derivative with respect to time of the current in direction β R the ohmic resistance p the pole pair number ω the rotational speed of the rotor ψ_(mα) the magnetic flux in the direction α ψ_(mβ) the magnetic fluix in the direction β u_(α) the voltage in direction α u_(β) the voltage in the direction β. wherein with the evaluations, the energy conditions of the rotor (2) are also taken into account.
 2. A method according to claim 1, wherein the energy conditions in the magnet (5) of the rotor (2) are taken into account by way of the following energy equations of the rotor: {dot over (ψ)}_(mα) =−p·ω·ψ _(mβ)  Equation (3) {dot over (ψ)}_(mβ) =p·ω·ψ _(mα)  Equation (4) wherein {dot over (ψ)}_(mα) is the derivative with respect to time of ψ_(mα) and {dot over (ψ)}_(mβ) the derivative with respect to time of ψ_(mβ).
 3. A method according to one of the preceding claims, wherein the motor model defined by the equations (1) to (4) is corrected in dependence on a comparison between computed model values ({circumflex over ( )}) and measured electrical and/or mechanical values by way of at least one correction term (9), so that there results the following equations: L·{dot over (i)} _(α) =−R·i _(α) +p·ω·ψ _(mβ) +u _(α)+υ_(1α)  Equation (1a) L·{dot over (i)} _(β) =−R·i _(β) −p·ω·ψ _(mα) +u _(β)+υ_(1β)  Equation (2a) {dot over (ψ)}_(mα) =−p·ω·ψ _(mβ)+υ_(2α)  Equation (3a) {dot over (ψ)}_(mβ) =p·ω·ψ _(mα)+υ_(1β)  Equation (4a) in which υ_(1α), υ_(1β), υ_(2α), υ_(1β) are correction terms
 4. A method according to claim 3, wherein the measured electrical values are the motor currents.
 5. A method according to one of the preceding claims, wherein the correction terms (9) are in each case formed from a correction factor and the difference between measured and computed motor currents.
 6. A method according to one of the preceding claims, wherein the correction terms (9) in the equations (3a) and (4a) in the one phase are formed by way of the difference between measured and computed currents of the other phase, wherein the correction term is introduced into equation (3a) with a negative polarity.
 7. A method according to one of the preceding claims, wherein the rotational speed is detected sensorically.
 8. A method according to claim 7, wherein the rotational speed is determined with the help of a Hall sensor.
 9. A method according to one of the preceding claims, wherein the rotational speed is evaluated by calculation in a manner such that the difference between the flux speed and an assumed rotor speed or variables derived therefrom is formed as a rotational speed correction term (11) and the actual [current] rotational speed is evaluated by way of an approximation process.
 10. A method according to claim 9, wherein the rotational speed correction term (11) is corrected by way of a rotational speed measurement.
 11. A method according to one of the preceding claims, wherein the assumed rotor rotational speed by way of a rotational speed correction term (11) is adapted in an adapter block (10) to the actual [current] rotational speed.
 12. A method according to one of the preceding claims, wherein the assumed rotational speed by way of a rotational speed correction term (11) is adapted in a rotational speed model to the actual rotational speed.
 13. A method according to one of the preceding claims, wherein for evaluating the flux speed one determines the position of the magnetic flux and specifically by way of the equation $\begin{matrix} {\rho = {\frac{1}{p} \cdot {{Arctg}\left( \frac{\psi \quad m\quad \beta}{\psi \quad m\quad \alpha} \right)}}} & {{Equation}\quad (5)} \end{matrix}$


14. A method according to claim 13, wherein the equation (5) is differentiated with respect to time and the equations (3a) and (4a) (for calculated evaluation of the rotational speed) are substituted into the differentiated equation (5).
 15. A method according to claim 12, wherein in the rotational speed model the derivative with respect to time, preferably of the first order, of the rotational speed is used.
 16. A method according to one of the preceding claims, wherein the rotational peed model is formed by a mechanical condition equation preferably of the form: $\begin{matrix} {{\overset{.}{\omega} = {\frac{1}{J} \cdot \left( {M - M_{L}} \right)}},} & {{Equation}\quad (8)} \end{matrix}$

in which M is the driving moment, M_(L) the load moment, and J the moment of inertia of the rotating load.
 17. A method according to claim 16, wherein the load moment is set to zero.
 18. A method according to claim 17, wherein the drive moment is set to zero.
 19. A method according to one of the preceding claims, wherein the load moment is formed by M _(L) =K ₁·ω²,   Equation (11) in which K₁ is a constant.
 20. A method according to one of the preceding claims, wherein the drive moment is defined by M=K ₂·(ψ_(mα) ·i _(β)−ψ_(mβ) ·i _(α)),   Equation (10) in which K₂ is a constant. 